Wherever it is necessary to transfer torque between shafts where the axes of the shafts intersect, but are not coaxial, it is common practice to use a device referred to as a "universal joint." Where only a slight angle of displacement between the two shafts is involved, it is possible to use relatively simple devices with either a lost-motion connection, or which accommodate the articulation with resilient members. With larger angles of shaft displacement, however, a more sophisticated system becomes necessary. A typical and very common application of this type of mechanism occurs at the points of transfer of torque at the opposite ends of the automobile drive shaft, where one end is driven by the output shaft of the transmission, and the opposite end delivers power to the differential assembly. The vertical movement of the axes of the rear wheels with respect to the frame results in a varying relationship between the points of torque transfer, and requires the installation of universal joints to accommodate this motion.
The most common form of universal joint capable of substantial angular deviation between the driving and driven shafts is the so-called "Cardan joint." Essentially, this device consists of a cross-shaped member, with one set of diametrically-opposite points being pivotally connected to the opposite ends of a fork secured to a driving shaft, and the other diametrically-opposite points received by a similar fork secured to a driven shaft. The points of pivotal interconnection between the forks and the cross member are usually provided with needle bearings to accommodate the articulation and the relatively high forces transferred at these points. The problem with the Cardan joint, however, is that the rotation of the driven shaft does not precisely follow the rotation of the driving shaft. Because of the geometry involved, there is a continuing angular oscillation of the one shaft with respect to the other, the amount of which is related to the deviation angle between the driving and driven shafts. At low speeds, this usually presents very little problem; but at high speeds, these torsional variations produce oscillations that generate objectionable vibration. Much attention has been devoted to the design of a universal joint that would not display these variations between the rotation of the driving and driven shafts. This type of joint has been designated as a "constant-velocity" universal joint, and a number of these devices have been worked out with limited success. One form of such a unit involves inner and outer spherical members with opposite channels arranged in planes generally parallel to the axes of the shafts, with these channels formed on the inner surface of the outer spherical member, and vice versa, so that the two sets of channels form passages which entrap a set of balls responsible for the transfer of forces between the inner and outer members. Some sort of positioning cage interposed between the spherical members is usually necessary to locate the balls so that their centers fall on a common plane, which usually is the plane perpendicular to the plane of the shaft axes, and bisecting the angle between the driving and driven shafts. This plane is conveniently referred to as the "bisector plane." The need for a locating cage for the force-transfer balls necessarily results in a substantial radial distance between the points at which forces are applied to the balls by the inner and outer spherical members. The greater this distance, the more intense are the bearing forces at these points of contact, which results from the angular relationship of the force vectors. If the points of force transfer could be maintained at positions where the force vectors were all tangential, the local bearing forces would be brought to a minimum. This desirable situation is not associated with any previous constant velocity joint I am aware of.